A desirable feature for ion-exchange media used in applications such as fuel cells, batteries, sensors, electrolyzers and other catalysis systems is the ability to deliver the highest exchange surface area while minimizing the size and weight of the entire system. An important metric used in comparing the performance of different system designs is the ratio of the exchange area to the volume of the system. For example, in a fuel cell, the increased contact area between the electrolyte, reactants and the catalytic surface results in an increase in the number of reactions per unit time. Therefore, the development of methods to increase surface area is critical to the improvement of technologies dependant on ion exchange. Common methods of increasing surface area fall into one of three categories, namely, microfibers, porous materials and roughened or microtextured surfaces.
With regard to the last category, a well known method for producing roughened surface on a nano scale is the plasma process. The process, however, requires high temperature and pressure that may damage certain substrates. Other methods of roughening include the impingement of sand or other particulates against a surface or the use of abrasives mounted on substrates; grinding wheels and sandpaper are examples. These processes, however, only provide limited surface area enhancement and are fraught with problems associated with contamination.
In catalysis systems, such as fuel cells, batteries, sensors, and electrolyzers, the ion exchange membrane is typically coated with a continuous or discontinuous layer of catalyst to promote the rates of chemical reactions. Commonly used catalysts include platinum (Pt) and Pt alloys, vanadium (V) and V alloys, titanium dioxide, iron, nickel, lithium and gold.
A fuel cell is an electrochemical apparatus wherein chemical energy generated from a combination of a fuel with an oxidant is converted to electric energy in the presence of a catalyst. The fuel is fed to an anode, which has a negative polarity, and the oxidant is fed to a cathode, which, conversely, has a positive polarity. The two electrodes are connected within the fuel cell by an electrolyte to transmit protons from the anode to the cathode. The electrolyte can be an acidic or an alkaline solution, or a solid polymer ion-exchange membrane characterized by a high ionic conductivity. The solid polymer electrolyte is often referred to as a proton exchange membrane (PEM).
The simplest and most common type of fuel cell employs an acid electrolyte. Hydrogen is ionized at an anode catalyst layer to produce protons. The protons migrate through the electrolyte from the anode to the cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of fuel cell are shown in the following equations:Anode reaction (fuel side):2H2→4H++4e−  (I)Cathode reaction (air side): O2+4H++4e−→2H2O  (II)Net reaction: 2H2+O2→2H2O  (III)
The goal is complete hydrogen oxidation for maximum energy generation shown in the equation. However, the oxidation and reduction reactions require catalysts in order to proceed at useful rates. Catalysts are important because the energy efficiency of any fuel cell is determined, in part, by the overpotentials necessary at the fuel cell's anode and cathode. In the absence of an catalyst, a typical electrode reaction occurs, if at all, only at very high overpotentials.
One of the essential requirements of typical fuel cells, and indeed any ion exchange system, is easy access to the electrode and a large surface area for reaction. This requirement can be satisfied by using an electrode made of an electrically conductive porous substrate that renders the electrode permeable to fluid reactants and products in the fuel cell. To increase the surface area for reaction, the catalyst can also be filled into or deposited onto a porous substrate.
However, these modifications result in a fragile porous electrode that needs additional mechanical support. An alternative is to sinter a porous coating on a solid substrate and then fill or re-coat the porous coating with a catalyst. The sintering process, however, is a multiple step procedure that requires baking at high temperatures.
In U.S. Pat. No. 6,326,097 to Hockaday, a surface replica technique is used to form an “egg-crate” texture on a membrane in a micro-fuel cell. The catalyst and metal electrode are applied to the surface of the membrane, and then the membrane is etched away so that the catalyst and electrode surfaces replicate that texture. This procedure is complicated, requiring blind etching and many separate operations.
Others have used silicon micro machining to increase the effective surface area of an electrode (Lee, S. J. et al., Miniature Fuel Cells with Non-Planar Interface by Microfabrication. In: Power Sources for the New Millenium, Jain, M. et al. (eds.), Proceedings Volume 2000-22, The Ion exchange Society Proceeding Series, Pennington, N.J., 2000). Etching of silicon is a very time-consuming process.